Amazing lens, incredible sharpness

Anders W wrote:

Great Bustard wrote:

thk0 wrote:

Ah. I see I misread your post. You are only making a point about total light gathered, not how it might translate into IQ.

Actually, I'm saying equivalent lenses will put the same total amount of light on the sensor for a given shutter speed, and that this will result in the same noise for equally efficient sensors.

I was questioning whether eg 4x more total light (ff at same f-stop) gives exactly 4x less noise.

4x as much (2 stops more) light will result in 1/2 as much (1 stop less) photon noise. The other player is read noise (the additional noise added by the sensor and supporting hardware).

The read noise becomes dominant only in the portions of the photo made with very little light -- most of the noise in the photo is dominated by photon noise.

True. But a) the portions of the photo made with very little light are those where quality shortcomings are most visible...

Only if they are pushed way up, such as with a very strong tone curve or very high ISOs.

...and b) the lower the overall light level, the greater the portion of the photo where read noise makes its presence known. Both consideration are of considerable importance, in my opinion.

Let's quantify that.  The EM5 has a QE of 53% and a read noise of around 2.5 electrons per pixel at the higher ISOs, which means the read noise matters more than photon noise for signals less than 12.5 photons per pixel, which is 6 stops below full "saturation" at ISO 6400, or 2.3 stops above the noise floor.

So, yes, read noise most certainly matters more and more as the light dims, but the photon noise is still dominant for most of the photo until you get to outrageously low levels of light (e.g. much lower light than the conditions you would use ISO 6400 for).

That's not to say that read noise is irrelevant -- perish the thought! -- but it is to say that it is of secondary, not primary, importance for the vast majority of lighting situations.  In fact, dare I say it, read noise is all but irrelevant for modern sensors at and below ISO 1600, and likely a couple of stops lower light still.